1. Field of the Invention
The present invention relates to a method and apparatus of purifying exhaust gas discharged from internal combustion engines, and more particularly to an exhaust gas purifying method and apparatus which is adapted to reduce detrimental components, i.e., nitrogen oxides, carbon monoxide and hydrocarbons, contained in exhaust gas with high efficiency.
2. Description of the Prior Art
Heretofore, there have been proposed various methods for reducing detrimental components, i.e., nitrogen oxides, carbon monoxide and hydrocarbons, contained in exhaust gas of internal combustion engines. Feedback control using an oxygen sensor can be cited as one of the methods practically employed in internal combustion engines for vehicles, etc. According to this method, the oxygen content of exhaust gas discharged from an internal combustion engine is detected by the oxygen sensor to judge whether an air-fuel ratio A/F is on the lean or rich side with respect to the theoretical A/F, and a signal is sent to a control unit such as a computer in accordance with the judgment to increase or reduce an amount of fuel, so that the actual A/F is controlled within a narrow range about the theoretical A/F.
But, an exhaust gas purifying catalyst has a purification characteristic variable dependent on the kinds of catalytic metals and reaction temperatures of catalytic metals. For example, therefore, in a catalytic system having such a characteristic that offers higher purification capability when variations in A/F are set larger with a catalytic layer ranging in lower temperatures and set smaller with the temperature rising up, it was impossible to achieve sufficient activity of an exhaust gas purifying catalyst from the range of lower temperatures by making use of the conventional systems.